Tesla Motors CEO Elon Musk thinks that it’s an “open secret” that Apple is working on developing its own electric car. “It’s pretty hard to hide something if you hire over a thousand engineers to do it,” Musk said in an interview with BBC News.

Officially, Apple hasn’t said a word publicly about developing an automobile. But reports have at least pointed to the project. In May, Apple was scouting for locations to test vehicles, according to the Guardian. In September, the company gave the heads of its “Project Titan” permission to triple the staff, up from the 600 people originally working on the program, according to the Wall Street Journal.

Apple in December also registered several car-related web domains, fueling the speculation around its project, according to MacRumors.

Despite Apple’s accelerated push into developing an Apple Car, Musk welcomed the entrance of additional players into the electric car market. "Tesla will still aspire to make the most compelling electric vehicles, and that would be our goal, while at the same time helping other companies to make electric cars as well,” he told the BBC on Monday.

But while his words were kind in his latest interview, Musk in the past has poked fun at Apple as the place where Tesla engineers go if they couldn’t make the cut. "They have hired people we’ve fired,” Musk said in an interview with German newspaper Handelsblatt in October. “We always jokingly call Apple the ‘Tesla Graveyard’. If you don’t make it at Tesla, you go work at Apple . I’m not kidding.”

Apple’s car isn’t expected until 2019 at the earliest, but in the meantime Tesla has made strides with its electric vehicles by introducing new features such as a self-driving mode and, most recently, the ability for a Tesla vehicle to self-park and pull out of the garage at its owner’s command via a feature called Summon.